1. Field of the Invention
The present invention relates generally to electromagnetic (EM) telemetry, and, more particularly, to a method and apparatus for facilitating EM wave reception of drilling and geological data in a fixed downhole receiver of an EM telemetry system. The invention has general application in the field of hydrocarbon exploration and production.
2. Description of Related Art
In standard practice, EM telemetry systems transmit drilling and geological data from downhole tools, such as a measurement-while-drilling (MWD) tool, to a location at the surface for analysis. The drilling and geological data usually provides important information regarding any potential problems that may occur during downhole operations. For example, the data characterizing the downhole conditions may indicate the production of water or sand, in which case, immediate notification of such is desired in order that corrective action may be taken. Accordingly, it is important to receive this downhole data at the surface in an accurate and expeditious manner to optimize operational response to any potential problems.
Currently, EM telemetry is generally limited to shallow land rigs where the formations are quite resistive (i.e., on the order of ten ohm-m or more). In a conventional EM telemetry system, an MWD tool includes a transmitter to transmit drilling and geological data to a receiver, which is typically located on the surface near the drilling rig. The transmitter of the MWD tool broadcasts a low frequency EM wave, typically in the tens of Hz or less. For a shallow and relatively high resistive formation, the current EM transmission scheme will typically suffice for conveying this data to the drilling surface.
In an offshore drilling operation, however, the EM wave will typically pass through thousands of feet of low resistivity formations of about 1 ohm-m, and then through hundreds to thousands of feet of salt water, having a resistivity of about 0.2 ohm-m, before reaching the receiver on the surface. Under the current EM telemetry scheme, however, the attenuation of the EM wave is too high for this approach to be practical. Moreover, the receiver being located on the drilling surface is typically subjected to high ambient EM noise from the drilling rig itself, thus further complicating the matter.
GB 2299915B to K. Babour (assigned to the present assignee) describes an alternative approach to placing the EM receiver at the surface. Babour proposes placing an EM receiver on the riser or on the platform itself. Even in such cases, however, the received EM signal might be quite small because of a likely low resistivity in those rock formations near the seabed. The method of Babour has been modified in U.S. Pat. No. 6,018,501, to Smith et al., to transmit the received EM signal from the seabed via an acoustic retransmission to a surface receiver. EP 0945590 A2 to Harrison proposes receiving the signal along an electrical conduit from a seabed template for transmission to the surface. Neither of these proposed techniques addresses the issue that the original EM signal received is small. Because it is important to receive the drilling and geological data accurately and expeditiously at the surface to take immediate corrective action for any problems that may occur during the downhole operations, an EM telemetry scheme which relies upon receivers near or above the seabed will not suffice for accomplishing such while drilling in deeper formations.
The present invention is directed to overcoming, or at least reducing the effects of, one or more of the problems set forth above.
In one aspect of the invention, a downhole telemetry system is provided. The system includes a first tubular disposed within a borehole, the first tubular having an elongated body and including at least one slot formed in a portion thereof. A second tubular is also disposed within the first tubular. The second tubular having a receiver, adapted to receive a signal, mounted thereon and positioned within the first tubular such that the receiver is aligned with the at least one slot formed in the first tubular.
In another aspect of the invention, a method for downhole telemetry is provided. The method includes disposing a first tubular within a borehole, the first tubularlincluding at least one slot formed in a portion thereof. A second tubular is disposed within the first tubular, the second tubular having at least one receiver mounted on its outer surface. The second tubular is positioned within the first tubular such that the at least one receiver is aligned with the slotted portion of the first tubular, and a signal is received at the at least one receiver.
In another aspect of the invention, a system for downhole telemetry is provided. The system includes a first tubular disposed within a borehole and a second tubular being disposed within the first tubular, the second tubular having a wireline attached to its outer surface. A receiver is provided and adapted to receive a signal, the receiver being mounted on the outer surface of the first tubular. A first coupler is mounted on the outer surface of the first tubular and connected to the receiver, and a second coupler is mounted on the outer surface of the second tubular and connected to the wireline. The first coupler is adapted to transfer the signal received by the receiver to the wireline via the second coupler.
In another aspect of the invention, a method for downhole telemetry is provided. The method includes mounting a first inductive coupler and a receiver on the outer surface of a first tubular, the first inductive coupler and receiver being connected to each other. A second inductive coupler is mounted on the outer surface of a second tubular. The second tubular is disposed within the first tubular, and a first signal is received at the receiver and transferred from the first inductive coupler to the second inductive coupler.
In another aspect of the invention, a method for downhole telemetry is provided. The method includes mounting a first inductive coupler and a transceiver on the outer surface of a first tubular, the first inductive coupler and transceiver being connected to each other. A second inductive coupler is mounted on the outer surface of a second tubular. The second tubular is disposed within the first tubular, and a first signal is received at the transceiver and transferred from the first inductive coupler to the second inductive coupler.
In another aspect of the invention, a system for downhole telemetry is provided. The system includes a first tubular disposed within a borehole, the first tubular having an elongated body and including an insulated gap formed in a portion thereof. A second tubular is disposed within the first tubular, the second tubular having a receiver mounted on its outer surface. An electrical coupling mechanism is further provided and adapted to electrically couple the first tubular to the second tubular. The second tubular is positioned within the first tubular such that the electrical coupling mechanism is positioned above the receiver on the second tubular and the receiver is positioned above the insulated gap formed in the first tubular.
In another aspect of the invention, a method for downhole telemetry is provided. The method includes disposing a first tubular within a borehole, the first tubular having an elongated body and including an insulated gap formed in a portion thereof. A second tubular is disposed within the first tubular, the second tubular having a receiver mounted on its outer surface. The second tubular is positioned with the first tubular such that the receiver mounted on its surface is positioned above the insulated gap formed in the first tubular. The first tubular is electrically coupled to the second tubular, with the electrical coupling occurring above the receiver mounted on the second tubular.